A microphone bias (micbias) block, or circuit, provides a regulated low noise voltage to an analog microphone. In applications such as cell phones, conventionally there are several microphones that are independently biased by dedicated micbias blocks. The microphones in such applications are typically placed far from each other resulting in significant ground mismatch between them. A conventional high-performance micbias block has an output pin (e.g., MIC1_BIAS in FIGS. 1 and 2) and a filter pin (e.g., MIC1_BIAS_FILT in FIG. 1 and MICBIAS_FILT in FIG. 2). The filter pin requires a large capacitor (e.g., 4.7 μF in metric 0402 size, e.g., C_EXT_FILT1 in FIG. 1 and C_EXT_FILT in FIG. 2) for each micbias block, primarily to meet low noise and high Power Supply Rejection Ratio (PSRR) requirements. The filter pin is referenced to the local microphone ground which helps with system level noise rejection. A dedicated filter capacitor requirement for each micbias block may add significant area and cost to the end application.
Referring now to FIG. 1, a diagram illustrating a prior art microphone bias scheme employing an external capacitor for each of N microphones is shown. A voltage reference, MIC1_BIAS_FILT, with respect to chip ground, CHIP_GND, which is remote/non-local to a microphone MIC1 102-1, is generated using a bias current IB1 104-1 and a resistor RBG1. The voltage reference MIC1_BIAS_FILT is filtered using a large dedicated external capacitor, C_EXT_FILT1, referenced externally to a local microphone ground, MIC1 GND_REF. The voltage reference MIC1_BIAS_FILT is used as an input to a driver stage amplifier AMP1 112-1 to drive the remote microphone element MIC1 102-1. An external capacitor C_MIC_EXT1 is used as a decoupling capacitor for the remote microphone element MIC1 102-1. The circuit just described is replicated N times with N dedicated external filter caps, C_EXT_FILT1 to C_EXT_FILTN, for an application using N remote microphones MIC1 102-1 to MICN 102-N. For an application using N remote microphones 102, each microphone MICx 102-x has a unique and dedicated MICx_BIAS_FILT voltage.
Disadvantages of the prior art scheme of FIG. 1 are that it uses a dedicated external filter capacitor C_EXT_FILTx for each microphone MICx 102-x. The dedicated filter capacitor requirement for microphone bias has the following system implications. The filter capacitors use up significant circuit board area, which limits the number of possible microphones in an area-constrained application, such as a cell phone, and adds to system level cost.
Referring now to FIG. 2, a diagram illustrating a possible prior art microphone bias scheme that shares an external filter capacitor, C_EXT_FILT, between multiple microphones 202 is shown. Three microphones MIC1 202-1, MIC2 202-2, and MIC3 202-3 are shown in FIG. 2. The scheme of FIG. 2 is similar to FIG. 1, except the MICBIAS_FILT voltage is not unique for each microphone MICx 202-x. The input to the driving stage amplifiers AMP1-AMP3 212-1 to 212-3 is a shared voltage, MICBIAS_FILT. In a system using multiple remote microphones, the scheme of FIG. 2 cannot generate a constant bias voltage across the remote microphones MICx 202-x because the MICBIAS_FILT voltage is not generated with respect to the local ground references of each microphone MICx 202-x. Additionally, the scheme of FIG. 2 cannot generate a unique bias voltage for each microphone MICx 202-x because the input to driving stage amplifiers AMP1-AMP3 212-1 to 212-3 is common.
To reiterate, a disadvantage of the scheme of FIG. 2 is that it does not allow independent control of microphone bias voltages and is unable to generate a unique voltage for each microphone. Further, the scheme of FIG. 2 cannot generate a constant bias voltage across the remote microphones. Finally, it may not be possible to meet system level ground noise rejection requirements with the scheme of FIG. 2.
As mentioned above, certain applications, such as cell phones, require multiple high-performance microphones, which are biased using low noise micbias circuits. Conventional solutions typically use dedicated external filter capacitors for noise filtering of each micbias instance as shown in FIG. 1. The dedicated filter capacitor requirement for microphone bias may have system implications. First, the filter capacitors may use up significant board area. For example, the filter capacitor area for six micbias instances in current technology may be approximately 9 square millimeters (e.g., 4.7 ρF in metric 0402 size). Additionally, the area consumed by the external filter capacitors may limit the number of possible microphones in the system. Finally, the external filter capacitors add to system level cost.